Joining 6 .stl files together to make 1 file to go to a comercial printer
Yes, you can.   Use Blender to import and position all of your stl files.

Group select all of the objects (so that they are highlighted).


Use Object -> Join to make all of the individual items one object. The Object menu option appears at the lower left of the Object Mode screen -- if you are in Edit or another mode, you won;t see it...


Making sure the object is highlighted, use File -> Export -> Stl


You'll end up with an STL composed of multiple non-connected parts. Whether this is cheaper or not is an issue for your printer: It might take as long to rip and print, and will certainly require the same amount of material to output -- infact, maybe more.... since I am not sure whether the method you are using will add supports or connected sprues between the separate items that were originally the separate stls... But this process will give you an stl with multiple 'pieces'.

Shout if I can assist.